In the dispensing of minute, precise quantities of liquid, such as the dispensing of epoxy to encapsulate portions of integrated circuits, it is important to achieve and maintain high repeatability in the dispensing quantity, notwithstanding possible variations in the temperature and/or viscosity of the liquid. For some applications, the liquid dispensed is extremely sensitive to such changes. For instance, in encapsulating integrated circuits, it is typical to use a two component epoxy which is premixed by the epoxy manufacturer and then frozen. The epoxy must then be used within a few days, and in some instances within several hours.
For typical dispensing operations, an epoxy for encapsulating of this type will have a relatively high viscosity. However, as the temperature changes, the viscosity is also subject to change. These viscosity variations can affect dispensing volumes, particularly if a positive volume displacement device is used to dispense the epoxy. For a given set of dispensing stroke parameters, i.e., displacement distance, force and rate, the dispensing result will vary with variations in viscosity.
Another problem relates to air or bubble entrapment within the liquid to be pumped from the pumping device. Obviously, if a displacement pump is compressing entrapped air during the displacement strike, the relationship between the displacement stroke and the dispensed volume will become distorted.
Most of the premixed two part epoxies used for encapsulations of this type contain an abrasive fill material, such as silica. This abrasive filler can cause undue wear on dispensing apparatus, again adversely affecting dispensing and/or resulting excessive downtime due to the need to repair or replace worn components.
While auger pumps have been used in the past to perform precise quantity dispensing of this type, the output of a typical auger pump varies directly with variations in the viscosity of the dispensed fluid, the needle size and the supply pressure. While the effect of viscosity changes and the fluid flow rate can be minimized with proper pump design, advanced calibration techniques and manual adjustments of the valve operation are required. Thus, the maximum possible mass flow rate while maintaining accuracy is limited.